A phased array is a group of radio frequency antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. In typical embodiments, they incorporate electronic phase shifters that provide a differential delay or phase shift to adjacent radiating elements to tilt the radiated phase front and thereby produce far-field beams in different directions depending on the differential phase shifts applied to the individual elements.
A number of embodiments of delay lines and antenna elements can be arranged in an RF antenna assembly. The antenna assembly may include an array of antenna elements. Such arrays of antenna elements may, in certain embodiments, be spatially arranged in either a non-uniform or uniform pattern to provide the desired antenna assembly characteristics. The configuration of the arrays of antenna elements may affect the shape, strength, operation, and other characteristics of the waveform received or transmitted by the antenna assembly.
The antenna elements may be configured to either generate or receive RF signals. The physical structure of the element for signal generation and reception is similar, and typically a single element is used for both functions. A phase shifter/true time delay (PS/TTD) device is a crucial part of the antenna element providing a differential delay or phase shift to adjacent elements to tilt the radiated/received phase front.
The active phased array antenna architecture is the most applicable to the use of the PS/TTD device. A schematic of one of the embodiments of an active phased array antenna unit is shown in FIG. 1. The antenna element is connected to a circulator, which is used to separate the high power transmit path and the low power receive path, providing the required isolation. The receive path includes a limiter to avoid damage from a high input level, followed by a low noise amplifier (LNA) used to bring the received signal up to the required power level. The output of the LNA passes through a transmit/receive switch, and then through the phase shifter/true time delay (PS/TTD) device, which provides the correct phasing for that element before the output is summed with that from all other elements. The PS/TTD provides the correct phase shifting of each antenna element at all frequencies. The overall phased array antenna output power is a coherent addition of the signals from each of the antenna elements. A large number of elements provide a large total power for the system.
The tunable delay application is not limited to active phased array antennas. Alternatively, PS/TTDs can be implemented in passive phased array systems, where the power is shared passively between many antenna elements, each having its own PS/TTD device.
Photonics technologies offer significant advantages over RF and microwave electronics, which can be exploited in phased array systems. Optics offer tremendous inherent bandwidth for use in optical processing and communicating systems, due to the very high carrier frequencies (e.g. 200 THz) compared to the microwave signals (10 s GHz) upon which they operate. Photonic technologies offer much lower cost if efficiently integrated. Photonic devices are inherently small due to the short wavelength at which they operate (around 1 micron) compared to the cm and mm wavelengths of microwave integrated circuits in phased array systems. Photonic integration provides a path to massive parallelism, providing additional reductions in size and weight, together with the promise of much lower overall system cost.
Phased array antenna using photonic delay lines is shown in FIG. 2. The laser emits coherent optical radiation with optical carrier frequency to ω0 into the optical fiber that takes it to the optical modulator where it gets modulated with RF signal containing RF frequencies Ω. The resulting optical signal contains frequencies ω=ω0±Ω where the information is carried (so-called signal sidebands) as well as remaining unmodulated laser carrier. This process is sometimes referred to as upconversion.
The optical signal next gets spitted between individual elements, each element containing photonic delay line, detector and the antenna. At the detector the optical signal of frequency ω gets down converted back to the RF of frequency Ω. Coherent addition of RF signals with different delays results in directional emission at angle θ.
This invention relates to optical delay lines based on microresonator structures. One of the most promising delay line designs is a ‘side-coupled integrated spaced sequence of resonators’ (SCISSOR) shown in FIG. 3(a). SCISSOR structures are by definition all-pass filters with light propagating in only one direction, and thus they have zero reflection. U.S. Pat. No. 7,058,258 discloses an implementation of the side-coupled sequence of resonators for tunable dispersion compensation. It provides different group delays at different frequencies of the optical signal. The present invention addresses an opposite goal—to achieve exactly the same group delay over as wide range of frequencies as possible.
Another configuration (FIG. 3(b)) of the side-coupled sequence of resonators was presented in U.S. Pat. No. 7,162,120, where the resonators are coupled to the opposite sides of the core waveguide. This configuration was designed only for device compactness; there is no performance difference between having resonators on one side or on both sides of the waveguide.
A multitude of phased array systems are used in many applications, varying from large surveillance systems to weapons guidance systems to guided missiles, plus many civil applications including weather monitoring radar systems, radio-astronomy and topography.
There is a need to provide more reliable and efficient devices for tunable delays to control phased array antennas. In the phased array antenna applications each frequency component of optical signal ω is down converted into an RF frequency component of angular frequency Ω with a phase delay ΦRF(Ω). The angle at which the phased array will emit the RF signal can be written as θ=sin−1(cωRF(Ω)/Ωd), where c is the speed of light and d is the distance between antenna elements.
In order to maintain the emission angle frequency-independent, it is required that ΦRF(Ω)/Ω=Td where Td is referred to as the true time delay that must be constant over the whole signal bandwidth. In the state of the art phased arrays the true time delay can be achieved only by using long propagation length, and it cannot be tuned easily. In this invention we propose a compact true time delay line that is also tunable over a wide range.